DBT using macros within external schema yml file

We define external redshift spectrum table using an yml file. Like the one below.

      - name: visitor
    description: 'visitor data'
    external:
      location: "s3://{{ env_var('ENV') }}-temp-raw-s3/{{ s3_prefix({{ env_var('ENV') }}) }}/Visitor"
      file_format: parquet
      partitions:
        - name: year
          data_type: VARCHAR(10)
          vals: # macro w/ keyword args to generate list of values
            macro: dbt.dates_in_range
            args:
              start_date_str: '{{ var("START_DT") }}'
              end_date_str: '{{ var("START_DT") }}'
              in_fmt: '%Y-%m-%d'
              out_fmt: '%Y'

I would like to change the prefix of the s3 bucket based on the environment variable.

Here is the macro I have tried to change the prefix. I am calling that macro in in the yml file above in line 4 (location).

{% macro s3_prefix(env_var) %}
{% if env_var == "prod" %}
    prefix_string = "prefix_prod"
{% else %}
    prefix_string = "prefix_non_prod"
{% endif %}

{{ return(prefix_string) }}                                                         
{% endmacro %}

I am getting some compilation errors. Is there an easier way to achieve this? Any help would be really appreciated.